MEMS acoustic sensor with integrated back cavity

ABSTRACT

A MEMS device is disclosed. The MEMS device comprises a first plate with a first surface and a second surface; and an anchor attached to a first substrate. The MEMS device further includes a second plate with a third surface and a fourth surface attached to the first plate. A linkage connects the anchor to the first plate, wherein the first plate and second plate are displaced in the presence of an acoustic pressure differential between the first and second surfaces of the first plate. The first plate, second plate, linkage, and anchor are all contained in an enclosure formed by the first substrate and a second substrate, wherein one of the first and second substrates contains a through opening to expose the first surface of the first plate to the environment.

FIELD OF THE INVENTION

The present invention relates to generally to MEMS devices, and moreparticularly, to a MEMS microphone.

BACKGROUND

Most commercially available MEMS microphones or silicon microphones areformed by two chips, an application specific integrated circuit (ASIC)chip and a MEMS chip attached to a substrate. These chips are generallyenclosed by a conductive cover or lid. An acoustic input can be providedfrom an opening on a top surface of the microphone or from an opening onthe substrate. Typically, in commercial applications where the acousticinput is from the top, an acoustic back cavity is formed mainly by avolume under the MEMS chip and the substrate. By contrast, in commercialapplications where the acoustic input is from the bottom, the acousticcavity is typically formed by the volume enclosed by the substrate andthe cover.

It is desirable to provide improvements to MEMS microphones which allowthem to be more easily manufactured at a lower cost. The improvement tothe MEMS microphone must be easily implemented, cost effective andadaptable to existing manufacturing processes.

The present invention addresses such a need.

SUMMARY OF THE INVENTION

A MEMS device is disclosed. The MEMS device comprises a first plate witha first surface and a second surface; and an anchor attached to a firstsubstrate. The MEMS device further includes a second plate with a thirdsurface and a fourth surface attached to the first plate. A linkageconnects the anchor to the first plate, wherein the first plate andsecond plate are displaced in the presence of an acoustic pressuredifferential between the first and second surfaces of the first plate.The first plate, second plate, linkage, and anchor are all situated inan enclosure formed by the first substrate and a second substrate,wherein one of the first and second substrates contains a throughopening to expose the first surface of the first plate to theenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures illustrate several embodiments of the inventionand, together with the description, serve to explain the principles ofthe invention. One of ordinary skill in the art readily recognizes thatthe particular embodiments illustrated in the figures are merelyexemplary, and are not intended to limit the scope of the presentinvention.

FIGS. 1A and 1B show different embodiments of the top view of the devicelayer of a torsional microphone.

FIG. 2A shows the cross section of the torsional microphone withintegrated back cavity along 2A-2A in FIG. 1A.

FIG. 2B shows the cross section of the torsional microphone withintegrated back cavity along 2B-2B in FIG. 1B.

FIGS. 3A and 3B show the operation of the torsional microphone using asymbolic representation for the linkage with torsional compliance

FIG. 4 shows an embodiment of the top view of a device layer of a pistonmicrophone.

FIG. 5 shows the cross section of the piston microphone with integratedback cavity along 5-5 in FIG. 4.

FIGS. 6A and 6B show the operation of a piston microphone using asymbolic representation for the linkage with bending compliance.

FIG. 7 shows alternative manufacturing options for a torsionalmicrophone.

FIG. 8 shows alternative manufacturing options for a piston microphone.

FIGS. 9A, 9B and 9C show packaging schemes for the current invention.

FIG. 10 shows an example of integration of MEMS microphone with otherMEMS device.

DETAILED DESCRIPTION

The present invention relates generally to MEMS devices, and moreparticularly, to a MEMS acoustic sensor such as a microphone. Thefollowing description is presented to enable one of ordinary skill inthe art to make and use the invention and is provided in the context ofa patent application and its requirements. Various modifications to thedescribed embodiments and the generic principles and features describedherein will be readily apparent to those skilled in the art. Thus, thepresent invention is not intended to be limited to the embodiments shownbut is to be accorded the widest scope consistent with the principlesand features described herein.

In the described embodiments Micro-Electro-Mechanical Systems (MEMS)refers to a class of structures or devices fabricated usingsemiconductor-like processes and exhibiting mechanical characteristicssuch as the ability to move or deform. MEMS devices often, but notalways, interact with electrical signals. MEMS devices include but arenot limited to gyroscopes, accelerometers, magnetometers, pressuresensors, microphones, and radio-frequency components. Silicon waferscontaining MEMS structures are referred to as MEMS wafers.

In the described embodiments, the MEMS device may refer to asemiconductor device implemented as a micro-electro-mechanical system.The MEMS structure may refer to any feature that may be part of a largerMEMS device. The semiconductor layer with the mechanically active MEMSstructure is referred to as the device layer. An engineeredsilicon-on-insulator (ESOI) wafer may refer to a SOI wafer with cavitiesbeneath the silicon device layer or substrate. A handle wafer typicallyrefers to a thicker substrate used as a carrier for the thinner silicondevice substrate in a silicon-on-insulator wafer. A handle substrate anda handle wafer can be interchanged.

In the described embodiments, a cavity may refer to an opening orrecession in a substrate wafer and an enclosure may refer to a fullyenclosed space. A post may be a vertical structure in the cavity of theMEMS device for mechanical support. A standoff is a vertical structureproviding electrical contact.

In the described embodiments, a back cavity may refer to a partiallyenclosed cavity equalized to ambient pressure via Pressure EqualizationChannels (PEC). In some embodiments, a back cavity is also referred toas a back chamber. A back cavity formed within the CMOS-MEMS device canbe referred to as an integrated back cavity. Pressure equalizationchannels, also referred to as venting or leakage channels/paths, areacoustic channels for low frequency or static pressure equalization of aback cavity to ambient pressure.

In the described embodiments, a rigid structure within a MEMS devicethat moves when subject to force may be referred to as a plate. Althoughrigid plates are preferred for the described embodiments, semi rigidplates or deformable membranes could replace rigid plates. Plates maycomprise of silicon, silicon containing materials (e.g. poly-silicon,silicon oxide, silicon nitride), metals and materials that are used insemiconductor processes (e.g. aluminum nitride, germanium). A back platemay be a solid or perforated plate comprising at least one electrode.The electrode can be comprised of semiconductor process compatibleconductive materials (e.g. poly-silicon, silicon, aluminum, copper,nickel, titanium, chromium, gold). The electrodes may have insulatingfilms on one or more surfaces.

In the described embodiments, perforations refer to acoustic openingsfor reducing air damping in moving plates. An acoustic port may be anopening for sensing the acoustic pressure. An acoustic barrier may be astructure that prevents acoustic pressure from reaching certain portionsof the device. Linkage is a structure that provides electricalconductivity and compliant attachment to a substrate through an anchor.Extended acoustic gap can be created by step etching of the post andcreating a partial post overlap over the PEC. In-plane bump stops limitrange of movement in the plane of the plate if the plates move more thandesired (e.g. under a mechanical shock). Similarly rotational bump stopare extensions of the plate to limit the displacement normal to theplane due to out-of-plane rotation.

In the described embodiments, structures (plates) of MEMS device andelectrodes formed on CMOS substrate form sensor capacitors. Sensorcapacitors are electrically biased for detection of change ofcapacitance due to acoustic pressure.

To describe the features of the present invention in more detail, refernow to the following description in conjunction with the accompanyingfigures.

FIGS. 1A and 1B show different embodiments of top views of device layers100A and 100B of torsional microphone. FIGS. 1A and 1B illustrates afirst plate 140, 142 that senses acoustic pressure on its first surface,and a second plate 150 with perforations 160 and a linkage 250, 252attached to an anchor 240, 242. In an embodiment the first plate 140,142 and second plate 150 are rigid. The difference between FIGS. 1A and1B are the locations of linkages 250, 252. A different embodiment mayinclude combination of linkages 250 and 252 resulting in four linkages,adding a central cutout portion to FIGS. 1A and 1B. The first plate 140,142 is partially surrounded by a pressure equalization channel (PEC)230, 232, and the device layer 100A, 100B is surrounded by a seal 260 toensure that the only acoustical input to the device will be via anacoustic port 190 (in FIGS. 2A and 2B.

When a force is applied (acoustic pressure variation) on the firstsurface of first plate 140, 142, the first plate 140, 142 isrotationally displaced around an axis passing through linkages 250, 252,hence the second plate 150 is displaced in an opposite direction(rotational displacement around the same axis). The linkages 250, 252form torsional restoring forces acting against movement and will bringthe plates to their initial position once externally applied acousticforce is zero. Undesired in plane movements can be limited byintroducing in plane bump stops 340 at locations where undesiredmovement/rotation has a high amplitude, e.g. furthest away from linkages250, 252. The in plane bump stops 340 can be defined and manufactured onthe second plate 150 or the device layer 100A, 100B or the first plate140, 142 or any combination of these.

In an embodiment, protruding tabs that form rotational bump stops 350are provided to limit the rotation of the first 140,142 and secondplates 150. By proper design the rotational bump stops 350 may eliminateneed for reduction or turning off the potential difference between firstand second plates 140, 142 and 150, and the electrode 170 shown in FIGS.2A and 2B for recovery from a tip-in or out of range condition.

FIGS. 2A and 2B show the cross section of the torsional microphone 200Aand 200B with integrated back cavity 130 along 2A-2A and 2B-2B in FIGS.1A and B respectively. In an embodiment, integrated back cavity 130 isformed by a fusion bond 220 between the second substrate 120 and thedevice layer 100A and 100B which is further bonded to the firstsubstrate 110 by conductive alloy (eutectic) bond 200 by processes asdescribed in a commonly owned U.S. Pat. No. 7,442,570, entitled, “Methodof Fabrication of a Al/Ge Bonding in a Wafer Packing Environment and aProduct Produced Therefrom”, which is incorporated herein by reference.

Static pressure in the back cavity 130 is equalized by ambient pressurevia air flow through the PEC 230 and 232. Ideally, PEC 230 and 232,provide high resistance to air flow in the frequency range of interest(e.g. 100 Hz and above), and low resistance at lower frequencies down tostatic pressure changes. Linkages 250 are attached to standoffs 180 bothmechanically and electrically. The standoffs 180 in an embodiment arelithographically defined protruding members of device layer that aremechanically and electrically connected to top conductive layers of thefirst substrate 110 via alloy or eutectic bonding. The device layer 100Aand 100B in an embodiment is lithographically patterned to form thefirst plate 140, a second plate 150, with perforations 160, PEC 230,232and an acoustic seal 260, around the active device.

The second plate 150 with perforations 160 forms a first electrode andis electrically connected to an integrated circuit (IC) manufactured onthe first substrate 110, while a second electrode 170 is disposed on thefirst substrate 110. Second electrode 170 is aligned with the firstelectrode or second plate 150. A first surface of second plate 150 andthe second electrode 170 form a variable capacitor whose value changesdue to pressure being applied on a first surface of first plate 140.142.In an embodiment, additional material such as silicon nitride or siliconoxide is deposited on the second electrode 170. The additional materialcan be lithographically patterned to form bump stops 270 to reducestiction force by reducing the contact area in the undesired event thatfirst and/or second plate 140,142 and 150 come into contact with firstsubstrate 110.

FIGS. 3A and 3B illustrate the conceptual design describing theoperation of the torsional microphone of FIG. 2A or 2B with a symbolicanchor 183, and a symbolic torsional linkage, 253.

Referring now to FIG. 3A, the acoustic port 193 is a channel in thefirst substrate 110 that allows acoustic pressure to reach the firstsurface of the first plate 143. Under an applied acoustic pressure, thefirst plate 143 rotates slightly either clockwise or counter-clockwisedepending on polarity of acoustic pressure. In FIG. 3B, the case wherethe first plate 143 rotates in a clockwise direction around a rotationaxis that coincides with linkage like structure 253 is depicted.

Rotational movement coupled to the perforated second plate 153 resultsin a reduced gap between first surface of the second plate 153 and asecond electrode 173, hence the capacitance defined by these twosurfaces increases. An IC manufactured on the first substrate 110 iselectrically connected to both the second plate 153 and second electrode173 detects the change in capacitance proportional to the acousticpressure.

FIG. 4 shows a top view of device layer 400 of a piston microphone withrigid first plate 144 that senses acoustic pressure on its firstsurface, a rigid second plate 154 with perforations 164, and linkages254 attached to an anchor 244. The number of linkages 254 shown in thedevice is four, but the number of linkages could be any number and thatwould be within the spirit and scope of the present invention. Undesiredin plane movements can be limited by introducing in plane bump stops 344at locations where undesired movement/rotation has a high amplitude,e.g., furthest away from the linkages 254. The in plane bump stops 344can be defined on the second plate 154 or the device layer 104 or thefirst plate 144, or any combination thereof.

FIG. 5 shows the cross section of the piston microphone 500, withintegrated back cavity 134 along 5-5 in FIG. 4. In an embodiment, thedevice layer 104 is device layer 400 in FIG. 4. The integrated backcavity 134 is formed by a fusion (oxide) bond 224 between a secondsubstrate 124 and the device layer 104 which further is bonded to thefirst substrate 114 by a conductive alloy (eutectic) bond 204 byprocesses as described in a commonly owned U.S. Pat. No. 7,442,570,entitled, “Method of Fabrication of a Al/Ge Bonding in a Wafer PackingEnvironment and a Product Produced Therefrom”, which is incorporatedherein by reference. Static pressure in the back cavity 134 is equalizedby ambient pressure via air flow through the PEC 234. Linkages 254 areattached to the standoffs 184 both mechanically and electrically.

Acoustic barriers 364 may be introduced wherever suitable for requiredlow frequency response enhancement.

The first plate 144 is partially surrounded by a PEC 234. The entirestructure is surrounded by a seal 264 to ensure that the only acousticalinput to a cavity 134 is via acoustic port 194. When an acoustic forceis applied on the first surface of first plate 144, the first plate 144is displaced up or down depending on polarity of pressure. The secondplate 154 is displaced in the same direction as the first plate 144.Both plates 144 and 154 are attached to the anchors 244 via the linkages254, which apply an opposite restoring force to first and second plates144 and 154. When the acoustic force is reduced to zero, the restoringforce brings first and second plates 144 and 154 to their originaloperating position.

The standoffs 184 are lithographically defined protruding members of thedevice layer that are mechanically and electrically connected to thefirst substrate 114 via alloy (eutectic) bonding to a top metal layer ofthe first substrate 114. The device layer 104 is lithographicallypatterned to form the first plate 144, second plate 154 and plate withperforations 164, the PEC 234 and an acoustic seal around the activedevice. The second plate 154 forms a first electrode and is electricallyconnected to an integrated circuit (IC) manufactured on the firstsubstrate 114, while a second electrode 174 manufactured on the firstsubstrate 114 is designed to be aligned with first electrode 174. Afirst (bottom) surface of the second plate 154 and the second electrode174 forms a variable capacitor whose value depends on the pressureapplied on the first surface of the first plate 144. The secondelectrode 174 in an embodiment is buried under a stack of siliconnitride and silicon dioxide which further can be lithographicallypatterned to form bump stops 274 to reduce stiction force by reducingcontact area in the undesired event that first and/or second plates 144and 154 come into contact with the first substrate 114.

FIGS. 6A and 6B illustrate the conceptual designs showing the operationof a piston microphone of FIG. 5. The linkages 254 in FIG. 5 are nowrepresented by symbolic springs 256 and support the first plate 146,second plate 156 the acoustic port 196 is a channel in a first substrate116 for acoustic pressure to reach the first surface of the first plate146. Under an applied acoustic pressure the first plate 146 slightlymoves up or down depending on polarity of sound pressure. In FIG. 6B,the case where the first plate 146 moves up is depicted. This upwardmovement of first plate 146 is coupled to a second plate 156 withperforations 166, which in turn results in increased gap between thefirst surface of the second plate 156 and the second electrode 176;hence the capacitance defined by these two surfaces decreases. An ICmanufactured on the first substrate 116 is electrically connected toboth of the electrodes 156 and 176; hence it is used to detect thechange in capacitance, which is proportional to the acoustic pressure.

FIG. 7 shows alternative manufacturing options for a torsionalmicrophone 700. In one alternative scheme, the posts 210 can be madewider to overlap over a PEC 230, while forming a shallow recess step toform a well-controlled and shallow extended PEC 280 for improving thelow frequency response of the microphone. The depth of the channel iscontrollable as well as the length to provide a means to properly designa pressure equalization channel for proper frequency response. Similarlydefining a partial overlap of the second substrate 120 over the outerperiphery of the second plate 150 creates a bump stop 310 which limitsout of plane, upward movement of the first and second plates 140 and150. By proper design of the bump stop 310 the potential risk of thefirst plate 140 touching the first substrate 110 can be reducedsignificantly. Similarly, proper design of the length of an extended PEC300 over outer edge (furthest away from the rotation axis) of the firstplate will limit the rotational movement of the first and second plates140 and 150 and may be used for significantly reducing the potentialrisk of first or second plates 140, 150 touching the first substrate110. Limiting out of plane movement improves device reliability,especially against stiction, vibrations and shocks.

In another embodiment, the first and second plates 140 and 150 can bethinned down selectively so as to have a thicker portion and a thinnerportion, creating a stepped device layer 290, for increasing resonantfrequency of the device and reducing acoustic resistance of theperforations 160. In an embodiment, linkage 250 can have the samethickness as the thicker portion of first plate 140 or second plate 150.In another embodiment, linkage 250 can be same thickness as the thinnerportion of first plate 140 or second plate 150. In another embodiment,linkage 250 can be of any thickness independent of the first and secondplates. By proper design of the step profile of the first and secondplates 140 and 150, first and second plates can be manufactured to bestiff enough to perform as microphone plates.

In another embodiment, back plate 330 with perforations 320 is providedto serve as a rigid electrode on the first substrate covering acousticport 190, which faces the first surface side of the first plate 140. Inan embodiment, the rigid back plate 330 can partially or completelycover the acoustic port 190. By proper design of a plate 330 withperforations 320, acoustic pressure input through acoustic port 190 willreach the first surface of the first plate 140 without noticeableattenuation, while the parallel plate capacitance formed by thisbackplate 330 and the first plate 140 will increase the electronic sensecapacitance.

Under the influence of acoustic input, the capacitance between thebackplate 330 and first plate 140 will change in the opposite phase tothe capacitance formed between the second plate 150 and the secondelectrode 170. The phase difference between sense capacitances enablesdifferential sensing. An additional benefit of the differentialstructure is the possibility of recovering from a stiction. In the eventthat either the first plate 140 or the second plate 150 comes intocontact with the first substrate 110 and gets stuck, an electrical biascan be applied between the plate that is not in contact with the firstsubstrate 110 and corresponding electrode (second electrode 170 or thebackplate 330) for recovering from stiction. It is also possible tosense the tilting of plates and dynamically adjust bias applied acrossthe plates to ensure that they do not come into contact with the firstsubstrate 110.

FIG. 8 shows alternative manufacturing embodiment for the pistonmicrophone. In one embodiment, the posts 214 can be made wider tooverlap over a PEC 234, while forming a shallow recess step to form awell-controlled and shallow extended PEC 284, in order to improve lowfrequency response of the microphone. In a similar way, a partialoverlap of bump stop 314 of the second substrate 124 over the outerperiphery of the second plate 154 limits out of plane (upward) movementof the first and second plates 144, 154. Limiting of out of planemovement improves device reliability, especially to vibrations andshocks.

In another alternative scheme, the first and second plates 144, 154 canbe thinned down selectively, creating a stepped device layer 294 toincrease resonant frequency of the structure and to reduce acousticresistance of perforations.

In another embodiment, backplate 334 with perforations 324 is providedto serve as an electrode on the first substrate covering acoustic port194, which faces the first surface side of the first plate 144. In anembodiment, the rigid back plate 334 can partially or completely coverthe acoustic port. By proper design of a plate 334 with perforations324, acoustic input (sound pressure) through the opening (acoustic port194) will reach the first surface of the first plate 144 withoutnoticeable attenuation, while the parallel plate capacitance formed bythis backplate 334 and the first plate 144 will increase the electronicsense capacitance.

Under the influence of acoustic input, this capacitance will change inthe same phase as the capacitance formed between the second plate 154and the second electrode 174. Hence the total sense capacitance willincrease.

FIGS. 9A, 9B, and 9C show packaging schemes for that can be applied toany of the described embodiments of a microphone. FIG. 9A illustrates acapped package 900A with integrated device 914. Back cavity 916 isself-contained in the integrated device 914. FIG. 9B shows a moldedpackage 900B where a plastic or similar encapsulating material 924 ismolded or formed over the integrated device 922. FIG. 9C illustrates acapped package 900C that forms an extended back cavity 927 via anacoustic port 926 opened on top surface of integrated device 918.

FIG. 10 shows an embodiment which integrates a MEMS microphone 370 withone or more other MEMS devices 380 on the first and second substrates.Other MEMS devices include but are not limited to the gyroscope,accelerometer, pressure sensor and compass. MEMS microphone 370 can be apiston microphone or a torsional microphone as described in FIGS. 1, 2,4, 5, 7, and 8.

Both torsional and piston designs of microphone provide improvementsover conventional designs. The integrated back cavity where theenclosure is defined by the first and second substrates and integratedelectronics from the CMOS-MEMS construction enables a significantlysmaller package footprint than in conventional two-chip solutions. Theintegrated back cavity also relieves packaging considerations where theMEMS die and package together form the back cavity.

The torsional design inherently is expected to be less sensitive toaccelerations during operation compared to similar dimensioned or largermicrophones. Piston design, in terms of electronic pickup and movementof plates, is similar to existing MEMS and condenser microphones, butunlike the others is based on movement of solid plates, not diaphragms.Also, unlike other designs, pressure sensing area and electrode area canbe adjusted separately, giving extra flexibility on design at a cost ofarea/mass.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A MEMS device comprising; an anchor attached to afirst substrate; a first plate with a first surface and a secondsurface; a second plate with a third surface and a fourth surface,attached to the first plate; and a linkage connecting the anchor to thefirst plate, where the first plate and second plate are displaced in thepresence of an acoustic pressure differential between the first andsecond surfaces of the first plate; wherein the first plate, the secondplate, the linkage, and the anchor are contained in an enclosure formedby the first substrate and a second substrate; wherein one of the firstand second substrates contains an acoustic port to expose the firstsurface to the environment, wherein at least one electrode is formed onthe first substrate.
 2. The MEMS device of claim 1, where each of thefirst and second plates comprises of a material containing silicon. 3.The MEMS device of claim 1, wherein the second plate is perforated. 4.The MEMS device of claim 1, wherein the second plate moves in anopposite direction of the first plate.
 5. The MEMS device of claim 4,wherein first and second plates are torque balanced around a rotationalaxis.
 6. The MEMS device of claim 1, wherein the second plate moves inthe same direction as the first plate.
 7. The MEMS device of claim 1,where the MEMS device comprises a microphone.
 8. The MEMS device ofclaim 1, where the enclosure forms an acoustic cavity.
 9. The MEMSdevice of claim 1, further comprising in-plane bump stops to limitlateral movement of the first and second plates.
 10. The MEMS device ofclaim 1, further comprising a capacitor, wherein a first electrode isformed by the second plate and a second electrode is formed by aconductive layer on the first substrate.
 11. The MEMS device of claim10, wherein the second electrode comprises of aluminum.
 12. The MEMSdevice of claim 10, wherein the second electrode is connected to anintegrated circuit on the first substrate.
 13. The MEMS device of claim1, further comprising a perforated plate disposed below the first plateand covering at least a portion of the acoustic port.
 14. The MEMSdevice of claim 1, wherein a gap is defined by a standoff etched intothe second substrate.
 15. The MEMS device of claim 1, further comprisingan additional insulating material disposed on the second electrode. 16.The MEMS device of claim 15, the additional insulating material isextended in select portions to form bump stops.
 17. The MEMS device ofclaim 1, wherein at least one bump stops is formed from a receded post.18. The MEMS device of claim 1, wherein the first and second plates haveuniform thickness.
 19. The MEMS device of claim 1, wherein the linkagehas a thickness that is substantially the same as the thickness of thefirst plate or the second plate.
 20. The MEMS device of claim 1, whereinthe first plate has a thinner portion and a thicker portion.
 21. TheMEMS device of claim 20, wherein the linkage has a thickness equal tothe thinnest portion of the first plate.
 22. The MEMS device of claim20, the linkage having a thickness greater than the thinnest portion ofthe first plate.
 23. The MEMS device of claim 1, wherein an air flowpath is provided from the first surface to the second surface.